Fatty acids are a principle metabolic fuel for liver, kidney and heart. Oxidation of a fatty acyl-CoA to the corresponding trans-2,3-enoyl CoA is catalyzed by each of the acyl-CoA dehydrogenases. We propose to study the three-dimensional structure of the general acyl-CoA dehydrogenase from pig liver mitochondria. This enzyme has recently been under intensive investigation and progress has been made in elucidating the catalytic and structural properties of the enzyme. Detailed structural information obtained from high resolution x-ray structure analysis will enable us to relate the chemical functions to the structure of the protein and to define the catalytic mechanism. This enzyme has a molecular weight of 180,000 and contains four identical subunits, each containing one equivalent of flavin adenine dinucleotide. The enzyme has been crystallized in a form suitable for three-dimensional x-ray structure analysis. The crystals diffract to high resolution and the asymmetric unit contains two monomers of the tetrameric enzyme molecule. An electron density map at 5.5 A resolution was calculated with phases determined from two heavy atom derivatives. It showed clear contrast between solvent and protein regions and the local two fold axis was readily visible. The molecule has 222 symmetry and approximate dimensions of 65 x 74 x 80 A. It is proposed initially to extend the structural analysis to 3.5 A resolution and then to 3 A analysis for elucidating the detailed mechanism of action of oxidation of acyl-CoA thioesters and transfer of electrons to the electron transfer protein, the physiological oxidant of the dehydrogenase in the mitochondria.